feat(cache): split system prompt into stable/dynamic blocks for independent caching

[martinffx]

Issue for this PR

Closes #5416, #5224
Related: #14065, #5422, #14203, #14743

Part of the stacked PR decomposition of the prompt caching fix (2 of 4). Adds cache token audit logging as observability foundation, the metrics logged here are what we use to verify cache hit improvements in subsequent PRs.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Splits the system prompt into stable (global instructions + global skills) and dynamic (environment + project instructions + project skills) blocks. The LLM adapter already supports this split structure for independent cache-key reuse, so stable blocks can persist across turns without re-encoding.

Gated behind OPENCODE_EXPERIMENTAL_SYSTEM_PROMPT_SPLIT (env var or OPENCODE_EXPERIMENTAL).

• Instruction.system() returns { global, project } instead of a flat array, separating global config dir instructions from repo-local ones.
• SystemPrompt.skills() returns { global, project } by scope.
• When the flag is on, the assembled prompt is { stable: [...globalInstructions, ...globalSkills], dynamic: [...env, ...projectSkills, ...projectInstructions] } instead of a flat array.
• Added OPENCODE_EXPERIMENTAL_CUSTOMIZE_SKILL flag and withStatics import to fix typecheck.

How did you verify your code works?

bun run typecheck — all 14 packages pass.

Screenshots / recordings

N/A — no UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Feat/system prompt split doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, here are the related PRs found (excluding the current PR #27377):

Related PRs:

1. fix(cache): improve Anthropic prompt cache hit rate with system split and tool stability #14743 - "fix(cache): improve Anthropic prompt cache hit rate with system split and tool stability"

   • Directly related: This PR implements the prompt cache improvements that depend on the system prompt split structure being introduced in PR feat(cache): split system prompt into stable/dynamic blocks for independent caching #27377.

2. feat(cache): add cache token audit logging behind OPENCODE_EXPERIMENTAL_CACHE_AUDIT #27007 - "feat(cache): add cache token audit logging behind OPENCODE_EXPERIMENTAL_CACHE_AUDIT"

   • Closely related: This PR addresses cache token audit logging, which aligns with the observability foundation mentioned in PR feat(cache): split system prompt into stable/dynamic blocks for independent caching #27377's description.

3. chore(cache): rebase prompt caching updates for #14743 #26875 - "chore(cache): rebase prompt caching updates for fix(cache): improve Anthropic prompt cache hit rate with system split and tool stability #14743"

   • Related: This is a rebase/maintenance PR for the prompt caching updates.

4. fix(session): cache messages across prompt loop to preserve prompt cache byte-identity #24842 and fix(session): cache messages across prompt loop to preserve prompt cache byte-identity #25367 - "fix(session): cache messages across prompt loop to preserve prompt cache byte-identity"

   • Related: These PRs address prompt caching improvements across sessions.

These PRs are part of the same stacked PR decomposition effort for the prompt caching fix, as mentioned in the current PR's description. They're not duplicates but rather complementary PRs in the same feature set.